The present invention relates to a method for dispensing a fluid from a pressure tank, whereby the introduction of a pressurized gas into the pressure tank is provided via a proportional valve that is disposed in an inlet line of the pressure tank, the measuring of the pressure of the fluid located in the outlet line is provided by a first pressure sensor, and the opening and closing of an outlet valve in the outlet line is provided for. The invention further relates to an apparatus for dispensing a fluid from a pressure tank, and includes a proportional valve that is disposed in an inlet line of the pressure tank for introducing a pressurized gas, a first pressure sensor in an outlet line of the pressure tank for measuring the pressure of the fluid located in the outlet line, and an outlet valve in the outlet line.
Such methods and apparatus are known, for example, in coating systems for the manufacture of CDs. With these systems, it is important for a uniform and continuous coating of the CDs that the pressure of the lacquer provided at the outlet or dispense valve preferably has a constant predetermined value. With the known systems, the pressure value measured at the first pressure sensor is utilized as an actual value for an adjustment of the proportional valve. However, this results in the problem that the pressure measured at the first sensor drops due to dynamic line and filter pressure losses as soon as the outlet valve is opened. Due to this drop in pressure, the pressure at the proportional valve is readjusted until the actual value again coincides with the prescribed desired value. In this connection, readjustment also takes place during a dispensing process, which leads to imprecision with regard to the applied quantity of lacquer. After the closing of the outlet valve, the dynamic line and filter pressure losses no longer have any effect, and the measured pressure at the first pressure sensor again rises to an increased value. The proportional valve must again be readjusted since the previously introduced gas, which is generally nitrogen, is vented; the readjustment takes place until the actual value again corresponds to the desired value.
These control or adjustment processes lead to fluctuations of the dosing volume as well as to a high consumption of nitrogen. Furthermore, concentration changes and possibly a crystallization of the lacquer dissolved in the solvent can result due to solvent evaporation and a discharge of the solvent vapor together with the nitrogen that escapes during the readjustment. In addition, continuous oscillations of the regulator occur during a tank filling state with critical residence gas volumes, which leads to a greater consumption of nitrogen.
It is therefore an object of the present invention to provide a method and apparatus for dispensing a fluid from a pressure tank, according to which a uniform and continuous dispensing of the fluid is ensured in a simple and economical manner.
The stated object is inventively realized with a method of the aforementioned type in that a desired or set pressure value is determined as a function of the measurement result of the first pressure sensor and is transferred to the proportional valve, and the gas pressure in the inlet line is measured with a second pressure sensor disposed between the proportional valve and the pressure tank and is transferred to the proportional valve. By the measurement of the gas pressure in the inlet line, and the transfer of the measurement results to the proportional valve, the frequent readjustment of the proportional valve described above during opening and closing of the outlet valve is avoided, since no dynamic line and filter pressure losses occur between the proportional valve and the second pressure sensor. This leads to a low consumption of nitrogen since no discharge of nitrogen occurs during closing of the outlet valve, as a result of which also a lower change of the dye concentration in the solvent is achieved. Furthermore, continuous oscillations during the regulation process are suppressed, since the control loop formed by the proportional valve in the second pressure sensor is not oscillatory. As a consequence of the determination of a set pressure value as a function of the measurement result of the first pressure sensor, and transfer of this value to the proportional valve, there is effected an automatic adaptation of the system to changeable disturbance variables, such as tank filling state and filter pressure losses, as a result of which a stable regulating condition is achieved without oscillations and a high dosing precision is also achieved.
Pursuant to one preferred specific embodiment of the invention, for the determination of the set pressure value only those measurement results of the first pressure sensor are used that were measured with the outlet valve opened in order to prevent pressure changes, which occur when the outlet valve is opened or closed, from influencing the determination of the desired or set value. For a constant dosing volume flow, only the pressure at the external sensor with the valve opened is relevant and of interest. In this connection, only those measurement results of the first pressure sensor are used that were measured after a specific period of time after the opening of the outlet valve in order that oscillations that occur during the opening will have no influence upon the determination. In order to achieve a uniform dispensing of the fluid, for the determination of the set pressure value a measurement result of the first pressure sensor determined over a measurement interval is used.
The set pressure value, in addition to being determined as a function of the measurement result of the first pressure sensor, is preferably determined as a function of the measurement result of the second pressure sensor in order to achieve a better uniformity and suppression of disturbance variables. A pressure difference between the inlet and outlet line is preferably measured, whereby in one specific embodiment of the invention, the set pressure value is determined as a function of the measured pressure differential.
Pursuant to a particularly preferred specific embodiment of the invention, the determination and/or transfer of the set pressure value is carried out only when the outlet valve is closed to order to ensure that during a dispensing process no change of the prescribed set pressure value, and a readjustment possibly connected therewith, occur.
Pursuant to a further specific embodiment of the invention, the filling state height of the pressure tank is determined as a function of the measurement results of the first and second pressure sensors in order to provide an automatic indication thereof and to be able to correct the thereby resulting disturbance variables during the determination of the set pressure value.
For an automatic indication of a filter state, the state of a filter located in the outlet line is preferably determined as a function of the measurement results of the first pressure sensor. From the automatic indication it can be determined when a filter change is necessary. In this connection, the filter state is preferably determined as a function of a difference of the measurement results of the first pressure sensor with the outlet valve closed and opened. During the determination of the filter state, preferably only those measurement results are used that were measured after conclusion of a predetermined period of time after the closing or after the opening of the outlet valve in order that oscillations that result during the closing or opening do not have an influence upon the determination.
The object of the present invention is realized with an apparatus of the aforementioned type in that a control unit is provided for the determination, as a function of the measurement result of the first pressure sensor, of a set pressure value that is to be provided to the proportional valve, and a second pressure sensor is provided between the proportional valve and the pressure tank for measuring the gas pressure in the inlet line and for transferring the measurement result to the proportional valve. With such an apparatus, the advantages described above in reference to the method are achieved. For a particularly simple and economical embodiment of the invention, the second pressure sensor is preferably integrated in the proportional valve.
Pursuant to a further advantageous specific embodiment of the invention, a differential pressure sensor is provided and is disposed between the inlet and the outlet line.